Method of making glasses resistant to alkali metal vapors



UNITED STATES PATENT OFFICE METHOD OF MAKING GLASSES RESISTANT TO ALKALIMETAL VAPORS Robert H. Dalton, Corning, N. Y., assignor to Corning GlassWorks, Corning, N. Y., a corporation of New York No Drawing. ApplicationNovember 6, 1934 Serial No. 751,792

3 Claims. (01. 91-70) This invention relates to glasses which are saltin a uniform layer. The glass surface may resistant to discoloration byhot alkali metal vaalso be wet by merely rinsing it with the soluporsand has for its object to impart resistance tion and the film ofsolution remaining thereon to normally non-resistant glasses. is thendried uniformly by means of a jet of dry 5 Ordinary glasses are subjectto a rapid dishot air. The glass with its dry coating is then 5coloration of their surface when heated to 250 heated, preferably in anelectric mufile at about C. to 350 C., or higher, in contact with alkali600 C., or as hot as possible without deforming metal vapors, as forexample, in the sodium vathe glass for about an hour or a timesufilcient por arc lamp. Such discoloration greatly deto fuse the alkalicompound into the glass sur creases the luminosity of the lamp and hasbeen face. Bulbs treated in this manner may be 10 the chief obstacle tothe successful development heated for fifteen hours at 300? C. incontact with of this efiicient light source. It has generally saturatedsodium vapor without appreciable disbeen considered that thisdiscoloration is in some coloration. manner dependent upon a high silicacontent in The thickness of the coating may be increased the glasswhereby the sodium causes reduction by increasing the concentration ofthe salt solu- 15 of some of the silica to metallic silicon. Varioustion and/or by prolonging the exposure of the glasses of abnormally lowsilica content have warm glass to the mist thereof, but if too thickbeen proposed as envelopes for such lamps to the coating is liable tocrack or craze on account overcome the difilculty. These special glassesof the difierence between its expansion coeificient possess undesirablecharacteristics, particularly and that of the glass and, also, is liableto be- 20 with respect to chemical stability and softness, comequitehygroscopic. I have been unable to which not only makes diflicult thefabrication of obtain an exact measurement of the thickness of tubes,bulbs and the like therefrom, by ordinary my films because in mostarticles which I have glass working methods, butalso renders suchexamined microscopically the film merges with articles easilysusceptible to atmospheric attack. the glass and it is diflicult to seeany sharp line It has further been proposed to cover and protect ofdemarcation between the film and the glass. these special'glassesagainst atmospheric attack However, the proper thickness may readily beby flashing them onto the inside of ordinary obtained by trial. stableglasses, but their working properties are It is uncertain whether or notthe protective so different from normal glasses that the relayer thusproduced consists entirely of the 30 sults have been unsatisfactory.original alkali compound which was used to I have discovered thatordinary stable glasses make the coating. The firing process willunwhich are normally discolored by contact with doubtedly bring about achange in the composihot alkali metal vapors may be rendered resisttion.This change may comprise a decomposiant to such discoloration by coatingthem with tion of the alkali compound itself or a reaction 5 a fused-onfilm of an alkali compound and that thereof with the air or the moisturein the air such films are usually transparent and when apor it mayinclude a reaction with the constituents plied in sufficiently thinlayers do not craze or of the glass. peel when the glass is subjected tothermal Although in preparing my protecting coating 40 change. I preferto use sodium hydroxide or sodium car- 0 Protecting films made inaccordance with my bonate as the alkali compound, I have also usedinvention may be prepared by treating the sura variety of other alkalicompounds, such as face of the glass which is to be exposed to thepotassium iodide, sodium nitrate, sodium phosaction of alkali metalvapors with a substance phate, lithium hydroxide, potassium hydroxide,

15 which on being heated will fuse to give a thin etc., all of whichproduce some of the benefits of 5 uniform layer of alkali compound overthe surface my invention. I am, therefore, convinced that of the glass.compounds of the other alkali metals, namely,

In preparing such films I preferably proceed rubidium and cesium, willalso produce some of as follows: The glass surface is wet by means ofthe benefits of my invention and will fall within a finely atomizedspray or mist of a 10-20% the scope of the appended claims. aqueoussolution of sodium hydroxide, or sodium In the claims I use the termalkali compound" carbonate. Other solvents may be used in lieu not onlyto refer to the salt before its applicaof water. At the same time theglass is heated to tion to the glass but also to define the film on atemperature just high enough to cause the mist the glass when finished.Films of alkali com- 5 to dry rapidly on the glass leaving the depositedpound, prepared in accordance with my invention, are not seriouslyafiected by atmospheric moisture, but in fabricating a lamp from a bulbso coated on its interior, it is desirable to direct the flame of theburner away from the interior of the bulb and to keep all parts of thebulb as warm as possible to avoid condensation.

While in the foregoing there has been shown and described the preferredembodiment of my invention, it is to be understood that -minor 10changes in the details of construction, combination and arrangement ofparts may be resorted to without departing from the spirit and scope ofthe invention as claimed.

Having thus described my invention, 15 claim is:

1. The method of treating glass to make it resistant to discoloration byhot alkali metal vapors, which includes wetting the surface of the glasswith a solution consisting of an alkali compound 20 and a solvent, andheating the glass to fuse the alkali compound to the glass in ahomogeneously attached substantially transparent layer which is thickenough to prevent discoloration of the what I resistant to discolorationby glass by the alkali metal vapor and then enough to resist crackingand crazing.

2. The method of treating glass to makeit resistant to discoloration byhot alkali metal vapors, which includes wetting the surface of the glasswith a solution consisting of sodium hydroxide and a solvent, drying thewet film uniformly and heating the glass to fuse the alkali thereto in asubstantially transparent layer which is thick enough to preventdiscoloration of the glass by the alkali metal vapor and thin enough toresist cracking and crazing.

3. The method of treating glass to make it hot alkali metal vapors,which includes wettingthe surface of the glass with a solutionconsisting of sodium carbonate and a solvent, drying the wet filmuniformly, and heating the glass to fuse the alkali thereto in asubstantially transparent layer which is thick enough to preventdiscoloration of the glass by the alkali-metal vapor and thin enough toresist cracking and crazing.

ROBERT H. DALTON.

